The invention relates to an implantable dosing device for the continuous, controllable release of medication in the human or animal body, comprising a medicine reservoir of variable volume and a liquid chamber which is tensionally connected to the medicine reservoir and the volume of which can be varied by liquid which is transported through an ion exchange diaphragm by electro-osmosis due to an electric field between two electrodes.
In a number of diseases it is necessary for patients to receive medication over extended periods of time. Examples of this are the administration of Insulin in the case of diabetes, corticosteroids in the case of rheumatic diseases or cytostatica in the case of cancer. These medicines have heretofore been delivered to the body of the patient predominantly either orally or by injection at certain time intervals. Such delivery of medication is therefore intermittent and is matched to the actual requirement of the patient only imperfectly. With many medicines the accurate dosage is, furthermore, very important, as excess as well as deficiency may have detrimental effects.
A number of devices have therefore been developed for delivering medication to the human or animal body, in order to permit better, more accurate dosing. The following systems in particular, are known:
Mechanical pumps; PA1 Pumping systems, in which a volume change is brought about by changing the state of a gas (decompression) or a liquid (evaporation); PA1 Pump systems, in which an osmotic pressure difference is utilized for the transport; PA1 Pumping systems, in which a volume is changed by electro-osmosis.
The main disadvantage of the first-named systems is that movable parts must be used, for example, gears and valves, which are subject to wear in operation and in most cases do not meet the requirements specified as to dosing accuracy, service life and tightness. These pumping systems have the further disadvantage that a relatively large amount of energy is required for their operation. While continuous release can be achieved by utilizing an osmotic pressure difference for transporting small volumes of medication, uniform release cannot be achieved thereby. In addition, an osmosis pump cannot be controlled without a suitable control valve.
For the continuous, controllable or regulated delivery of medicines to the human or animal body, a dosing device is also known which works according to the principle of electro-osmosis and comprises a medicine reservoir provided with an opening as well as means for varying the reservoir volume in the form of a liquid chamber of variable volume, which is tensionally connected to the medicine reservoir (German Offenlegungsschrift No. 2 239 432 or U.S. Pat. No. 3,894,538). The volume change of this chamber is preferably brought about by the penetration of liquid by electro-osmosis due to an electric field. For this purpose, two porous electrodes are provided, between which an ion exchange diaphragm is arranged.
Such a dosing device, which works by electro-osmosis, has the advantage that it requires neither mechanically movable parts nor valves, and operates perfectly quiet; in addition, it can be accommodated in a small volume and relatively little energy is required to pump small amounts of medication. The dosing device is therefore highly suited for implantation in the body of patients.
The electrodes of the known dosing devices can be supplied with external power, for example, from a battery. Platinum electrodes, in particular, are used for this purpose. However, such electrodes can be operated with only small currents since otherwise the water decomposition which takes place due to electrolysis leads to gas development. This is because the occurring reaction products diffuse off too slowly and can no longer be removed. It is also known to design the electrodes themselves to deliver current. For this purpose, zinc, cadmium or aluminum anodes and silver/silver chloride cathodes, for example, can be used. Such electrodes have the disadvantage, however, that they are consumed and therefore limit the length of operation of the electro-osmosis dosing device.
Instead of consumable electrodes, one can also use electrodes which are operated with substances of the body itself, in particular, glucose anodes and oxygen cathodes. With such electrode systems, current densities of about 1 to 5 .mu.A/cm.sup.2 can be achieved at present. However, as with the currently available ion exchange diaphragms, current densities of about 0.2 mA/cm.sup.2 are required for the transport of liquid. Thus, the electrodes would be required to have, for example, an area of about 55 cm.sup.2 in order to generate a current of 110 .mu.A which is required for transporting 2 .mu.l/hour. Electrodes of this size are hardly suitable for implantation, however. In addition, the current generation with the mentioned electrodes is dependent on the supply of glucose and oxygen. Particularly if the active areas of the electrodes are limited by the growth of connective tissue, the danger therefore exists that the attainable currents and the dosing capacity of the electro-osmosis pump may become still smaller.